How do I create a solid object/cube
Blender is the wrong type of software for this. Of course software exists for solid modeling, typically CAD software or simulation software.

With Blender and similar software you define the outside shape of objects (mesh modeling). When it comes to cutting them apart and make them look as if they are filled you would use boolean operators (either directly or as modifier) that simulate filled objects. They calculate new outside faces to make it look as if the inside of the object is filled. Or you would create the geometry and the faces by hand.

So what you want is not possible with Blender. It depends highly on your specific goal if you need to switch the software or if it is better to adapt your process to Blender.